In 1856, seabird guano was an in-demand fertilizer, so the the United States Congress passed The Guano Islands Act. This legislation was a precursor to American imperialism, as it allowed citizens to claim bird-crap-covered islands in the USA's name.

Says Columbia University law professor Christina Duffy Burnett of this scatological land grab:

The Guano Islands Act of 1856 arguably laid the legal groundwork for American imperialism. [...] Basically what happened was that in the first half of the nineteenth century, Europeans and Latin Americans figure out that the phosphate-rich deposits of seabird droppings that had accumulated on many small Pacific islands make spectacular fertilizer. The stuff is like magic, and farmers everywhere are suddenly clamoring to get their hands on some. There's a boom, the price skyrockets, the Peruvians more or less control the market, and supplies are short. Everybody is looking for new sources, there's tons of fake guano trading hands-it's chaos. Enter the US farm lobby. Farmers in the United States start pressuring Congress to pass some sort of legislation that will improve domestic access to this vital excrement. The result is the Guano Islands Act, legislation that authorized the United States to take control of a guano island if a citizen discovered it and undertook certain actions to take possession of it. [...]

[The Act passes, and] there are all these wildcatters and roughnecks throwing up the Stars and Stripes on little mounds of manure all over the world. In the end, more than seventy such islands are actually secured under the act, and many more are claimed (unsuccessfully, for one reason or another). But that's not the interesting part, really-although it's curious enough, and there are some great stories about what goes down on these islands: shanghaiing Polynesian laborers, piracy (of course), mutiny, etc. Some of the islands are still claimed by various shady types. Indeed, a rather mysterious gentleman contacted me some years ago in connection with his alleged title to an uninhabited guano island in the Caribbean.

You can read her full and incredibly fascinating discussion of the The Guano Islands Act of 1856 at Cabinet Magazine.